If you need a quick fix on one of your photos or images, and you don't want to install any additional software on your computer, you can use the online PhotoShop Express. Check Jose's review and links to the solution. It's free and it's very good.But there is another online service for modifying your pictures that I believe is also very good: It's called Picnik. You can find it on www.picnik.com.
1. You don't have to signup or register to upload and edit picture
2. Lots of basic tools like Auto fix, Rotate, Straighten, Crop, Resize, Fix exposure, Sharpen, Red-eye removal, ...
3. Lots of effects for your photos
4. You can put texts, shapes, frames on the images
5. You can edit curves and color levels (for premium version only)
6. lots and lots of other stuff
7. If you want to experiment with the tool, it has some sample images for you to try the tools
8. It can connect to Flickr, Picasa, FaceBook and other online photo services, or you just upload your photo
Below is a sample that I made in just a couple of minutes using auto fix, exposure fix, rotate-straighten, added shape, text, some radial effect and drop shadow border and resized image:
I recommend you to visit the site and try it yourself.